Prehistoric Lizard Had the Teeth of a Dinosaur

The Mesozoic is often called the Age of Dinosaurs. This terminology casts a long shadow over ancient life.

In museums, documentaries, and books, the terrestrial environments of the Mesozoic are often presented as being the sole domain of dinosaurs, with stands of conifers parting just enough to see titanic creatures snarl at each other. It’s easy to forget that other forms of life – like some weird little mammals – also thrived at the same time. If we’re really going to understand what life was like back then, we need the whole picture. That includes carnivores that competed with the dinosaurs. Strange teeth uncovered at a dinosaur-bearing site in Texas remind us that small, non-dinosaurian predators also carved out a living in the Mesozoic world.

No one yet knows exactly what the body of the predator looked like. Only teeth, uncovered in 82-80 million year old rock just outside Big Bend National Park, have been find so far. And at first glance, they look pretty similar to the slicing dentition of small dinosaurs found at the same site. But from the pattern of serrations and other subtle aspects of tooth anatomy, Big Bend National Park paleontologist Steven Wick and colleagues have concluded that the teeth actually came from a carnivorous lizard.

As far as Wick and colleagues were able to discern, the lizard was a varanoid. That’s the group that contains monitor lizards, helodermatids (the Gila monster and its relatives), and some extinct lineages such as the mosasaurs. The new lizard likely belonged to the monitor or helodermatid line, but, without more bones, it’s impossible to say for sure. All the same, the teeth are distinctive enough that Wick and coauthors have named them Dryadissector shilleri – Shiller’s dissector of forest nymphs.

What makes Dryadissector so unusual, though, isn’t the lizard’s tooth anatomy. It’s how common the fossil is at the site. Wick and colleagues counted nearly 13 complete teeth and an additional 46 fragments, far outnumbering all the predatory dinosaur teeth at the site combined. Varanoid lizards have been found at other Cretaceous microsites, but not in such abundance. What made the haunt of Dryadissector different?

A hint might come from the abundant mammal teeth found in the same matrix. The remains of small mammals are often rare at Cretaceous microsites, but, at the site that yielded Dryadissector, mammal teeth outnumber all reptile fossils nearly three to one. While predatory dinosaurs were certainly around – Wick and coauthors found the teeth of tyrannosaurs and deinonychosaurs, among others – the dinosaurs may not have been as numerous as elsewhere. If it’s a real signal, this absence of dinosaurs allowed mammals to thrive and, consequently, for a large lizard to be their primary predator.

How this unusual assemblage came to be isn’t clear. Perhaps, being further south than other studied sites, the local climate or makeup of the habitat was more amenable to lizards than to small dinosaurs. Or, as Wick and coauthors point out, the site is a little bit older than most other well-known Cretaceous microsites, and may represent a time when big lizards were important small carnivores before little, feathery dinosaurs took over. All the same, Dryadissector is a reminder that there was far more to prehistoric life than the biggest or most charismatic of creatures.

Reference:

Wick, S., Lehman, T., Brink, A. 2014. A theropod tooth assemblage from the lower Aguja Formation (Early Campanian) of West Texas, and the roles of small theropod and varanoid lizard mesopredators in a tropical predator guild. Palaeogeography, Palaeoclimatology, Palaeoecology. doi: 10.1016/j.palaeo.2014.11.018